Similar to Rinsed; it describes someone who has just been made to look like a fool. This can be for many reasons, such as of losing an argument, not coming up with a good comeback to an insult, losing a competitive game or generally messing up.
Its thought that its usage comes from the well-known wit of Chris Lambert, a CBBC host in the early 90's famous for his put-downs.
'Your mum's a slag!'
'Dude, that guy totally berted you'
'I lost at one touch one bounce'
'Yeah, you were berted pretty bad'
'Take one step closer and I will bert you'
'Its a fine line between looking cool and looking as if you've been berted'